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Using 3D radiative MHD simulations and Lyman-$\alpha$ transit calculations,we investigate the effect of magnetic fields on the observational signatures ofatmospheric escape in exoplanets. Using the same stellar wind, we vary theplanet's dipole field strength ($B_p$) from 0 to 10G. For $B_p<3$G, thestructure of the escaping atmosphere begins to break away from a comet-liketail following the planet ($B_p=0$), as we see more absorbing material aboveand below the orbital plane. For $B_p\geq3$G, we find a ``dead-zone'' aroundthe equator, where low velocity material is trapped in the closed magneticfield lines. The dead-zone separates two polar outflows where absorbingmaterial escapes along open field lines, leading to a double tail structure,above and below the orbital plane. We demonstrate that atmospheric escape inmagnetised planets occurs through polar outflows, as opposed to thepredominantly night-side escape in non-magnetised models. We find a smallincrease in escape rate with $B_p$, though this should not affect the timescaleof atmospheric loss. As the size of the dead-zone increases with $B_p$, so doesthe line centre absorption in Lyman-$\alpha$, as more low-velocity neutralhydrogen covers the stellar disc during transit. For $B_p<3$G the absorption inthe blue wing decreases, as the escaping atmosphere is less funnelled along theline of sight by the stellar wind. In the red wing (and for $B_p>3$G in theblue wing) the absorption increases caused by the growing volume of themagnetosphere. Finally we show that transits below and above the mid-discdiffer caused by the asymmetry of the double tail structure.

The Effects of Magnetic Fields on Observational Signatures of Atmospheric Escape in Exoplanets: Double Tail Structures